In a mink cell line that contains the Moloney murine sarcoma viral genome, glucocorticoids induce foci of morphologically transformed cells and increases in viral RNA and viral p 30 antigen. Removal of glucocorticoid is followed by a return to be the base line of all parameters. In the differentiated state, mouse teratoma cells permissive for murine leukemia viral expression cells in the undifferentiated state showed a post penetration block to viral expression. A novel species of mouse 30S RNA has been detected in several mouse cell lines and appears to represent defective endogenous mouse C-type retroviral genetic material. The Friend strain of spleen focus-forming virus was shown to be a recombinant between ecotropic murine type-C retrovirus and the env gene region of xenotropic type-C retrovirus. Unintegrated Harvey sarcoma viral DNA, a linear molecule of 4 x 10 to the 6th power daltons, can transform cells without helper virus or its proteins. DNA from AKR cells which contain unexpressed endogenous leukemia viral genomes is not infectious unless the recipient cells are treated with 5-iododeoxyuridine.